1. Field of the Invention
The present invention relates to an image forming apparatus for forming images on paper; and in particular, to an image forming apparatus having a medium sensor for judging the quality of paper.
2. Description of the Related Art
A prior art image forming apparatus is shown in JP-A-2000-37915. The image forming apparatus has a medium sensor for judging the quality of paper. The medium sensor emits light toward paper, receives reflection light therefrom, and judges the quality of paper such as premium grade paper, transparent paper, coated paper, photograph paper, etc., on the basis of the reflectivity of the paper, whereby the paper feeding speed and ink quantity are varied in compliance with the paper.
FIG. 5 is a side-sectional view showing a prior art image forming apparatus having a medium sensor. The image forming apparatus 1 has the lower part of its body covered by the lower part cover 2 and has the upper part thereof covered by the upper part cover 3. A paper feeding portion 4 is provided rearward of the upper part cover 3 via an opening 4a. The paper feeding portion 4 is formed to be integral with the lower part cover 2 and retains paper fed through the opening 4a. 
A printing portion 9 for printing on paper, which is fed from the paper feeding portion 4 by rollers (not illustrated), is disposed forward of the paper feeding portion 4. The printing portion 9 includes an ink cartridge 6 which is held for scanning in the paper width direction by a carrier frame 7. Paper feeding is carried out by rollers 8, and an image is formed on the paper by causing the ink cartridge 6 to scan.
A medium sensor 10 is disposed between the paper feeding portion 4 and the printing portion 9 opposite the paper feeding portion 4. The medium sensor 10 includes an arm portion 12 having an axial hole 10a and a detection portion 13 attached to the tip end of the arm portion 12.
As shown in the upper surface view of FIG. 6, the medium sensor 10 is pivotally supported through the axial hole 10a on the sensor shaft 5 attached extending from the side portion of the lower part cover 2 (Refer to FIG. 5). A pressing spring 11 is provided in the circumference of the axial hole 10a of the medium sensor 10. The medium sensor 10 is pressed into the direction of the arrow A by the pressing spring 11, and the detection portion 13 coheres paper fed into the paper feeding portion 4. The detection portion 13 has a light-emitting element and a light receiving element (both are not illustrated), and emits light to the paper and receives reflection light by its light receiving element, whereby it becomes possible to judge the quality of paper.
However, according to the above-described prior art image forming apparatus 1, the disposing angle of the medium sensor 10 with respect to paper varies, depending on the number of sheets of paper supplied into the paper feeding portion 4. Therefore, there is a problem in that the detection portion 13 cannot sufficiently cohere the paper and the accuracy in judgement on the paper quality is lowered.
Also, the turning center (axial hole 10a) of the medium sensor is disposed in the vicinity of the paper feeding portion 4. Thereby, the detection portion 13 of the medium sensor 10 moves roughly in the vertical direction with respect to the paper feeding portion 4, and the width of the disposing angle of the medium sensor 10 with respect to paper is devised to become smaller. Therefore, it becomes necessary to provide a sensor shaft 5 which is disposed opposite the paper feeding portion 4 in order to cause the turning center of the medium sensor 10 to come close to the paper feeding portion 4, wherein there is another problem in that the number of components is increased.